In a typical casting process, molten metal is poured into a pre-formed mould cavity which defines the shape of the casting. However, as the metal solidifies it shrinks, resulting in shrinkage cavities which in turn result in unacceptable imperfections in the final casting. This is a well known problem in the casting industry and is addressed by the use of feeder sleeves or risers which are integrated into the mould during mould formation. Each feeder sleeve provides an additional (usually enclosed) volume or cavity which is in communication with the mould cavity, so that molten metal also enters into the feeder sleeve. During solidification, molten metal within the feeder sleeve flows back into the mould cavity to compensate for the shrinkage of the casting. It is important that metal in the feeder sleeve cavity remains molten longer than the metal in the mould cavity, so feeder sleeves are made to be highly insulating or more usually exothermic, so that upon contact with the molten metal additional heat is generated to delay solidification.
After solidification and removal of the mould material, unwanted residual metal from within the feeder sleeve cavity remains attached to the casting and must be removed. In order to facilitate removal of the residual metal, the feeder sleeve cavity may be tapered towards its base (i.e. the end of the feeder sleeve which will be closest to the mould cavity) in a design commonly referred to as a neck down sleeve. When a sharp blow is applied to the residual metal it separates at the weakest point which will be near to the casting surface (the process commonly known as “knock off”). A small footprint on the casting is also desirable to allow the positioning of feeder sleeves in areas of the casting where access may be restricted by adjacent features.
Although feeder sleeves may be applied directly onto the surface of the mould cavity, they are often used in conjunction with a breaker core. A breaker core is simply a disc of refractory material (typically a resin bonded sand core or a ceramic core or a core of feeder sleeve material) with a hole in its centre which sits between the mould cavity and the feeder sleeve. The diameter of the hole through the breaker core is designed to be smaller than the diameter of the interior cavity of the feeder sleeve (which need not necessarily be tapered) so that knock off occurs at the breaker core close to the casting surface.
Breaker cores may also be manufactured out of metal. DE 196 42 838 A1 discloses a modified feeding system in which the traditional ceramic breaker core is replaced by a rigid flat annulus and DE 201 12 425 U1 discloses a modified feeding system utilising a rigid “hat-shaped” annulus.
Casting moulds are commonly formed using a moulding pattern which defines the mould cavity. Pins are provided on the pattern plate at predetermined locations as mounting points for the feeder sleeves. Once the required sleeves are mounted on the pattern plate, the mould is formed by pouring moulding sand onto the pattern plate and around the feeder sleeves until the feeder sleeves are covered and the mould box is filled. The mould must have sufficient strength to resist erosion during the pouring of molten metal, to withstand the ferrostatic pressure exerted on the mould when full and to resist the expansion/compression forces when the metal solidifies.
Moulding sand can be classified into two main categories. Chemical bonded (based on either organic or inorganic binders) or clay-bonded. Chemically bonded moulding binders are typically self-hardening systems where a binder and a chemical hardener are mixed with the sand and the binder and hardener start to react immediately, but sufficiently slowly enough to allow the sand to be shaped around the pattern plate and then allowed to harden enough for removal and casting.
Clay-bonded moulding sand uses clay and water as the binder and can be used in the “green” or undried state and is commonly referred to as greensand. Greensand mixtures do not flow readily or move easily under compression forces alone and therefore to compact the greensand around the pattern and give the mould sufficient strength properties as detailed previously, a variety of combinations of jolting, vibrating, squeezing and ramming are applied to produce uniform strength moulds, usually at high productivity. The sand is typically compressed (compacted) at high pressure, usually using a hydraulic ram (the process being referred to as “ramming up”). With increasing casting complexity and productivity requirements, there is a need for more dimensionally stable moulds and the tendency is towards higher ramming pressures which can result in breakage of the feeder sleeve and/or breaker core when present, especially if the breaker core or the feeder sleeve is in direct contact with the pattern plate prior to ram up.
The above problem is partly alleviated by the use of spring pins. The feeder sleeve and optional locator core (typically comprised of high density sleeve material, with similar overall dimensions to breaker cores) is initially spaced from the pattern plate and moves towards the pattern plate on ram up. The spring pin and feeder sleeve may be designed such that after ramming, the final position of the sleeve is such that it is not in direct contact with the pattern plate and may be typically 5 to 25 mm distant from the pattern surface. The knock off point is often unpredictable because it is dependent upon the dimensions and profile of the base of the spring pins and therefore can result in additional cleaning costs. The solution offered in EP-A-1184104 is a two-part feeder sleeve. Under compression during mould formation, one mould (sleeve) part telescopes into the other. One of the mould (sleeve) parts is always in contact with the pattern plate and there is no requirement for a spring pin. However, there are problems associated with the telescoping arrangement of EP-A-1184104. For example, due to the telescoping action, the volume of the feeder sleeve after moulding is variable and dependent on a range of factors including moulding machine pressure, casting geometry and sand properties. This unpredictability can have a detrimental effect on feed performance. In addition, the arrangement is not ideally suited where exothermic sleeves are required. When exothermic sleeves are used, direct contact of exothermic material with the casting surface is undesirable and can result in poor surface finish, localised contamination of the casting surface and even sub-surface gas defects.
Yet a further disadvantage of the telescoping arrangement of EP-A-1184104 arises from the tabs or flanges which are required to maintain the initial spacing of the two mould (sleeve) parts. During moulding, these small tabs break off (thereby permitting the telescoping action to take place) and simply fall into the moulding sand. Over a period of time, these pieces will build up in the moulding sand. The problem is particularly acute when the pieces are made from exothermic material. Moisture from the sand can potentially react with the exothermic material (e.g. metallic aluminium) creating the potential for small explosive defects.
WO2005/051568 (the entire disclosure of which is incorporated herein by reference) discloses a feeder element (a collapsible breaker core) that is especially useful in high-pressure sand moulding systems. The feeder element has a first end for mounting on a mould pattern, an opposite second end for receiving a feeder sleeve and a bore between the first and second ends defined by a stepped sidewall. The stepped sidewall is designed to deform irreversibly under a predetermined load (the crush strength). The feeder element offers numerous advantages over traditional breaker cores including:—
(i) a smaller feeder element contact area (aperture to the casting);
(ii) a small footprint (external profile contact) on the casting surface;
(iii) reduced likelihood of feeder sleeve breakage under high pressures during mould formation; and
(iv) consistent knock off with significantly reduced cleaning requirements.
The feeder element of WO2005/051568 is exemplified in a high-pressure sand moulding system. The high ramming pressures involved necessitate the use of high strength (and high cost) feeder sleeves. This high strength is achieved by a combination of the design of the feeder sleeve (i.e. shape, thickness etc.) and the material (i.e. refractory materials, binder type and addition, manufacturing process etc.). The examples demonstrate the use of the feeder element with a FEEDEX HD-VS159 feeder sleeve, which is designed to be pressure resistant (i.e. high strength) and for spot feeding (i.e. high density, highly exothermic, thick-walled, and thus high modulus). The feeder sleeve is secured to the feeder element via a mounting surface which bears the weight of the feeder sleeve and which is perpendicular to the bore axis. For medium pressure moulding there is the potential opportunity of using lower strength sleeves i.e. different designs (shapes and wall thicknesses etc.) and/or different composition (i.e. lower strength). Irrespective of the sleeve design and composition, in use there would still be the issues associated with knock off from the casting (variability and size of footprint on the casting) and need for good sand compaction beneath the feeder element. If the feeder element of WO2005/051568 were to be employed in medium-pressure moulding lines it would be necessary to design the element so that it collapses sufficiently at the lower moulding pressure (as compared to high pressure moulding) i.e. to have a lower initial crush strength. It would also be highly advantageous to use lower strength feeder sleeves (typically lower density sleeves). In addition to removing the cost penalty (associated with having to use high strength high density sleeves), this would allow the use of sleeves better suited to the individual application (casting) in terms of volume and thermophysical properties. However, when this was first attempted it was surprisingly discovered that the feeder sleeve suffered damage and breakages on moulding which if used for casting would have resulted in the casting suffering from defects.
An improved feeder element was therefore devised and described in WO2007/141466 (the entire content of which is also incorporated herein by reference) to extend the utility of collapsible feeder elements into medium pressure moulding systems while allowing the use of relatively weak feeder sleeves without introducing casting defects. This feeder element is similar to that described above in relation to WO2005/051568 but further includes a first sidewall region defining the second end of the element and a mounting surface for a feeder sleeve in use, the first sidewall region being inclined to the bore axis by less than 90°, and a second sidewall region contiguous with the first sidewall region, the second sidewall region being parallel to or inclined to the bore axis at a different angle to the first sidewall region whereby to define a step in the sidewall. As for the feeder element described in WO2005/051568, it was similarly found that such an arrangement was advantageous in minimising the footprint and contact area of the feeder element, thereby reducing the variability associated with knock-off from the casting.
To satisfy productivity requirements, automated greensand moulding lines have become increasingly popular, for the high volume and long run manufacture of smaller castings, e.g. automotive components. Automated horizontally parted moulding lines using a matchplate (pattern plate with patterns for both cope and drag mounted on opposite sides) are capable of producing moulds at up to 100-150 per hour. Vertically parted moulding machines (such as Disamatic flaskless moulding machines manufactured by DISA Industries A/S), are capable of much higher rates of up to 450-500 moulds per hour. In the Disamatic machine, one pattern half is fitted onto the end of a hydraulically operated squeeze piston with the other half fitted to a swing plate, so called because of its ability to move and swing away from the mould. Vertically parted mould machines are capable of producing hard, rigid flaskless greensand moulds, which are particularly suited for ductile iron castings. In such applications, sand is typically blown at a pressure of 2 to 4 bar and then compacted at a squeeze pressure of 10 to 12 kPa, with a maximum of 15 kPa being used in certain high demand applications.
Castings produced horizontally offer greater flexibility in terms of ease of manufacture and there are numerous application techniques available, with potential access to the entire pattern area allowing feeders to be placed as and where required. Castings produced vertically pose greater challenges to ensure that they are consistently sound, and feeding is typically restricted to the top or side feeders placed on the moulding joint line, which makes the feeding of isolated heavier sections very difficult.
There are essentially two types of feed requirements for any casting, including those produced in vertically parted moulds.
The first feeding requirement is modulus driven, whereby modulus is a proxy for the solidification time of the casting or section of casting to be fed. For this, the feeder metal has to be liquid for a sufficient time i.e. greater than that of the casting and or casting section, to enable the casting to solidify soundly without porosity and thus produce a sound defect free casting. For these applications, it is possible to use a standard rounded profile sleeve (with a feeder element such as those shown in WO2005/051568 and WO2007/141466). In particular, for high pressure vertically parted moulding lines, compressible feeder elements are required to give the necessary sand compaction between the base of the feeder element and the pattern surface, and it has been found that the compressible feeder elements such as those in WO2005/051568 and WO2007/141466 are suitable to give the necessary sand compaction together with consistently good feeder removal (small footprint and easy knock off).
The second feeding requirement is volume driven, i.e. there is a need to supply a certain volume of liquid metal to the casting. The volume is determined by several factors, primarily the casting weight and the liquid and solid metal shrinkage of the particular metal alloy. Another factor is ferrostatic pressure (effective height of the liquid metal feeder above the neck or contact with the casting), which is particularly important for castings produced in vertically parted moulds.
It is the volume requirement and the dimensional restrictions in vertically parted casting moulds that the present invention is primarily concerned with.